What's the small bit between the hose and the SPG for

[tbone1004]

@Akimbo threads yes, I haven't seen anything that says lube the o-ring.
That o-ring looks more like the valve was wrenched down vs. not being lubed.

From Luxfer, which should be done on all bottles.
"The application of a small quantity of lubricant to the valve threads is essential for
the proper care of the valve and cylinder. We recommend that only Dow Corning
Compound 111 be used on valve threads.
Only a very small amount of the lube is needed, and only applied at the bottom of
the valve threads. The lubricant will ride up the threads as it is installed. We only
recommend this lubricant since it has proven compatible with cylinders and valves,
doesn’t add any grease or oil to the cylinder, is safe to use, and helps prevent galvanic
corrosion."

There is no mention of lubricating the o-ring in their maintenance guide. As there is mention of lubrication on the threads, it is safe to assume that you don't lube that o-ring.
In the valve service manuals it mentions lubricating the bonnet o-rings, and again no mention of lubing the sealing o-ring, same safe assumption that since they mention lubricating other o-rings, if they wanted that one lubed, they would have told you to.
I seem to remember someone, may have been the PST manual stating not to lube that o-ring for fear of extrusion, but I can't find that manual right now.

 

[Akimbo]

SCUBA tank valves were installed with plumbers sealant (proper name ?)

"Pipe dope", at least in the US. It is still widely used for larger threads. There is also a Teflon pipe dope.

For other readers:
Pipe dope and Teflon tape are lubricants, not sealants. Both extrude on inadequately tightened or malformed tapered threads (NPT or CGA). Here is more history on Scuba valves: Scuba Cylinder Valve Installation, History in the US heading

 

[Akimbo]

That o-ring looks more like the valve was wrenched down vs. not being lubed.

Like you, my first thought was that the O-ring was oversized and was extruded from tightening the valve onto the cylinder. The O-ring was not leaking when I removed the valve but was completely dry and had a "set" from being pressurized for several years. The "set" made it hard to precisely measure the cross section but both Parker and I were confident it was the proper size. I have seen this kind of damage several times before. If I remember correctly, they were all on valves used on steel cylinders (US standard).

Parker was confident that the lack of lubrication allowed the O-ring to abrade and pinch between the seating surfaces when torqued past the hand-tight point. This is/was a Buna-N O-ring from the pre-Nitrox era.

For other readers:
The valve flange stands proud of metal-to-metal seating when hand tight. Torquing with a wrench (or worse, a hammer!) doubtless caused this damage to the unlubricated O-ring. The flange on the valve should be torqued to form a zero-gap metal to metal fit. Technically you can't overtighten a Scuba valve into a cylinder short of thread failure. It needs to be tight enough for that metal-metal contact plus not unscrew during normal handling, even when unpressurized. Tightening beyond that serves no purpose beyond making it unnecessarily hard to remove the valve.

Installed Scuba valve cross section, unpressurized. Cylinder wall not to scale. The O-ring gland profile is typical of a steel cylinder. Aluminum cylinders use a flat bevel/chamfer.​

 

[Schwob]

I should not ask, but can't resist:
So, @Sam Miller III , divers / watermen of your vintage had no iron deficiency issues then? (As surely no man worth his bacon would let the cold rolled steel rubbing bacon go to waste)

 

[WarrenZ]

I use o2 compatible grease on all my scuba gear that needs it. Even the orings on my camera. It costs a little more but I only keep one scuba oring lube on hand and a small tube lasts for a lot of orings.

 

[happy-diver]

tbone1004, seriously???

dude you really have to start reading less and using your brain more.

 

[Dan]

I think it depends on how you compress the o-ring. If you press squarely the seal face against the o-ring, I can see no lubrication needed. The o-ring would just forming a flat band ring around the seal face.

If you screw down the seal face against the o-ring, you will shear and stretch the o-ring against the seal face compromising the sealing surface due to rippling out of the o-ring contact area as you compress and drag the o-ring along the seal surface. Thin film of lubricant would relief the shearing force and smooth out the ripples.

 

[Akimbo]

I think it depends on how you compress the o-ring. If you press squarely the seal face against the o-ring, I can see no lubrication needed.

Lubrication would be less critical in this case but the O-ring still moves in groove with each pressure cycle so lubrication has value. See the illustrations in O-rings for Divers, 2: How do O-Rings Work?

The real issue is there is never a good reason not to properly lubricate an O-ring. "Proper" lubrication requires and adequate amount, (which is not much) and a lube that is compatible with the O-ring, pressure, and the media under pressure.

Lightly lubricating O-rings on adequately torqued cylinder valves have never caused O-ring extrusion, but not lubricating it has caused significant O-ring damage.​

A brief look at The Parker O-Ring Handbook reveals the allowable gap on O-ring glands for a given pressure and material. The gap on an an adequately torqued Scuba cylinder valve is effectively zero or metal to metal. Granted, a minute gap can exist and is determined by adding the two allowable machining flatness tolerances. However, that gap is far less than is allowed in a piston seal design.

 

[Dan]

Lubrication would be less critical in this case but the O-ring still moves in groove with each pressure cycle so lubrication has value. See the illustrations in O-rings for Divers, 2: How do O-Rings Work?

The real issue is there is never a good reason not to properly lubricate an O-ring. "Proper" lubrication requires and adequate amount, (which is not much) and a lube that is compatible with the O-ring, pressure, and the media under pressure.

Lightly lubricating O-rings on adequately torqued cylinder valves have never caused O-ring extrusion, but not lubricating it has caused significant O-ring damage.​

A brief look at The Parker O-Ring Handbook reveals the allowable gap on O-ring glands for a given pressure and material. The gap on an an adequately torqued Scuba cylinder valve is effectively zero or metal to metal. Granted, a minute gap can exist and is determined by adding the two allowable machining flatness tolerances. However, that gap is far less than is allowed in a piston seal design.

I was thinking of my yoke regulator connection to the tank valve. I don't remember seeing diver applying lubricant over the o-ring on the tank valve. We just slide the regulator seal face into the valve port hole & hand tighten the yoke. I've done it almost 800 times without any leaking problem.

 

[Akimbo]

I was thinking of my yoke regulator connection to the tank valve. I don't remember seeing diver applying lubricant over the o-ring on the tank valve. We just slide the regulator seal face into the valve port hole & hand tighten the yoke. I've done it almost 800 times without any leaking problem.

I can't remember being on a large day boat or liveaboard without at least one regulator to cylinder connection leaking a tiny stream of bubbles. That valve O-ring is easily the most commonly carried spare. For comparison, the lubricated O-rings inside the first stage see far more pressurization cycles and rarely leak -- including the dynamic O-rings in a piston regulator.